mem-hotplug: fix typo in Documentation/memory-hotplug.txt

Fix a trivial typo in Documentation/memory-hotplug.txt.
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/memory-hotplug.txt

@@ -149,7 +149,7 @@ For example, assume 1GiB memory block size. A device for a memory starting at
 (0x100000000 / 1Gib = 4)
 This device covers address range [0x100000000 ... 0x140000000)
 
-Under each memory block, you can see 4 files:
+Under each memory block, you can see 5 files:
 
 /sys/devices/system/memory/memoryXXX/phys_index
 /sys/devices/system/memory/memoryXXX/phys_device